Ultrafast dynamics and computational studies on diaminodicyanoquinodimethanes (DADQs).

Article pubs.acs.org/JPCB

Ultrafast Dynamics and Computational Studies on Diaminodicyanoquinodimethanes (DADQs) Marek Szablewski,*,† Mark A. Fox,‡ Fernando B. Dias,† Hannah Namih,† Edward W. Snedden,† Simon M. King,† DeChang Dai,† and Lars-Olof Pålsson*,‡ †

Centre for Materials Physics, Department of Physics, Durham University, South Road, DH1 3LE Durham, United Kingdom Department of Chemistry, Durham University, South Road, DH1 3LE Durham, United Kingdom

‡

S Supporting Information *

ABSTRACT: Three diaminodicyanoquinodimethanes, 4-(R1R2C)-1-[(NC)2C]-C6H4 (R1,R2 = H2N, 1; R1 = 3,5-Me2-4-OCH4H6N−, R2 = H2N, 2; R1 = 3,5-Me2-4OCH4H6N−, R2 = 4-Me-C5H9N, 3), were investigated using carbon-13 NMR, steadystate, and ultrafast transient absorption and ultrafast fluorescence spectroscopies to unravel the unusual characteristics of this class of chromophores. Computed (GIAO)B3LYP/6-31G* data for the zwitterions 1−3 using necessary solvation (PCM) models were shown to be in excellent agreement with observed structural and carbon-13 NMR data. The ground-state geometries of 1−3 contain a cationic methine group R1R2C− twisted from the C6H4 ring and an anionic methine group (NC)2C− in plane with the C6H4 ring in solution and solid state. The 13C chemical shifts of the peak corresponding to the methine carbon at the (NC)2C− group of 1−3 are observed at 32.5−34.7 ppm, which are some 55 ppm upfield compared with the 13C chemical shift for the methine carbons in TCNQ, 1,4-[(NC)2C]2-C6H4. The decay of the excited state in diaminodicyanoquinodimethanes is fast and dominated by nonradiative processes on the picosecond time scale, which depends on the viscosity of the medium. The dynamics of the excited-state decay is therefore limited by conformational changes through an intramolecular twisting motion. This twisting motion is hindered by friction, which, in turn, also depends on the functional group size of the system. The dominant nonradiative pathways after excitation are due to twisted excited-state conformers according to TD-DFT computations.

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INTRODUCTION Diaminodicyanoquinodimethanes (DADQs) with formulas 4(R1R2C)-1-[(NC)2C]-C6H4 such as 1−3 (Chart 1) are

liquid crystal displays, through to environmentally sensitive optical probes.16−19 Although the parent disubstituted dicyanoquinodimethane, in which two nitrile moieties are substituted with NH2 groups, (Chart 1, structure 1), was first reported in 1962,2 its optical properties and NMR spectroscopic data are presented here for the first time. The optical and electronic properties of functionalized DADQ adducts 2 and 3, (Chart 1) mainly in the condensed phase, have been investigated in our laboratory.3−5,14−18 Studies of such TCNQ substitution products have demonstrated their potential in, for instance, sensing applications, as the local environment strongly modulates the excited state and thereby the fluorescence.3,4,11−19 The motivation for considering DADQ adducts for these applications arises from the ease, by which DADQ can be modified through variations of functional groups. The prospect of design and definition of adducts with specific covalent or electrostatic interaction to almost any material is an attractive feature in itself worthy of exploration. In low viscosity media, the fluorescence from a DADQ adduct is very weak with photoluminescence quantum yields

Chart 1. Diaminodicyanoquinodimethanes (DADQs) 1−3 Investigated in This Work

conveniently synthesized from reactions of 7,7,8,8-tetracyanoquinodimethane (TCNQ, 1,4-[(NC) 2 C] 2 -C 6 H 4 ) with amines.1−12 Such compounds with varied functional groups can be easily modified into molecules designed for specific interactions with the surrounding media. DADQs have been investigated mainly for applications including nonlinear optics9,13 but also for electroluminescent devices,17 dye doped © 2014 American Chemical Society

Received: November 19, 2013 Revised: May 22, 2014 Published: May 27, 2014 6815

dx.doi.org/10.1021/jp411358d | J. Phys. Chem. B 2014, 118, 6815−6828

The Journal of Physical Chemistry B

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(PLQYs) in the range of 10−2 to 10−3. Increasing the viscosity or lowering the temperature results in a dramatically increased PLQY and increased fluorescence lifetime from the picosecond to the nanosecond time domain.5,18 Conformational change in the form of a rotation of the molecular planes relative to each other is therefore the main driving force of the excited-state decay. In viscous media or at lower temperatures, the rotation is hindered and the radiative excited-state decay is favored over the nonradiative pathway. The nonradiative rate is also dependent on size and steric factors, as these also influence the rotation. The impact of the physical size for different πextended branched systems with chemically very similar properties on the interplay between radiative and nonradiative decay channels has been demonstrated for a different class of organic materials.20 DADQs such as 1−3 contain unique features that put this group of materials in a class of its own. The generic structure consists of an electron-donating group, which can be varied in the specific derivative, and the electron-accepting dicyanomethylene subunit, −C(CN)2. Hence, the ground-state dipole moments in DADQs can be very large, μg = 15−20 D.3−5 In the excited state, however, the dipole moment is remarkably small, μe ≤ 0.5 D,3−5 and, to our knowledge, few materials exhibit this particular behavior, that is, μg > μe, where betaine adducts and merocyanines are among the few examples.21,22 The position of the absorption maximum in a DADQ is sensitive to the polarity of the medium, whereas the fluorescence maximum is considerably less sensitive.3,5 The latter observation indicates that there is a very weak solvent−solute interaction in the excited state and is rationalized by a small excited-state dipole moment, μe. What is particularly interesting in this context are hydrogen-bond interactions between solvent and solute. The versatile hydrogen-bond interactions are crucial for biological structures and functions.23−29 Hydrogen bonds have been intensively studied for model systems but usually for chromophores with larger excited-state dipole moment, as compared with the ground-state dipole moment, that is, μg < μe, where the dipole moment increases during electronic excitation. Hence the situation is sometimes complicated with hydrogenbond breaking and reformation occurring, in parallel to solvation effects during the electronic excitation event. With the unique electronic properties of functionalized DADQ adducts such as 1−3 (Chart 1), there is therefore an opportunity to study these extremely important interactions from a slightly different and possibly unique perspective. Recent reports on the ultrafast dynamics of TCNQ showed the dynamics to be extremely fast.30 Here we resolve the excitedstate dynamics of the DADQ adducts 1−3 through the use of both ultrafast fluorescence up-conversion (UPC) and transient absorption (TA) measurements. In parallel, computations were carried out on 1−3 to examine their molecular geometries, electronic structures, rotation barriers, 13C NMR data, and their excited states.

stated. Because all of the materials considered in this work have relatively large ground-state dipole moments, μg, as previously discussed, aggregates such as physical dimers could be formed at higher concentrations. These would have different optical properties when compared with the monomeric (nonaggregated) system. It is usually accepted that, in particular, TA, but also UPC measurements, requires more concentrated sample solutions than conventional fluorescence measurements. However, when comparing both absorption and fluorescence of the samples used in the experiments of this study, with extensively diluted samples, no differences in the spectra were observed. Little or no presence of physical ground-state aggregates in the sample materials is assumed here. TA and UPC measurements were accordingly performed with lowest possible concentrations, typically in the 10−6 M range. Another critical aspect concerning the laser spectroscopy experiments of this work is the impact of high-excitation intensities. To establish if there was any impact fluorescence as well as the UPC, we measured decays in an excitation range, which was varied by two orders of magnitude, from the photon flux. Ultrafast Transient Absorption Spectroscopy. Ultrafast TA measurements were performed using a conventional femtosecond noncolinear pump−probe setup. 180 fs, 5 μJ, pulses of 100 kHz, and repetition rate at 780 nm (1.60 eV) were generated using a Coherent Mira900-f Ti:sapphire femtosecond oscillator in conjunction with a Coherent RegA 9000 laser amplifier. The pulses were fed into a Coherent 9400OPA, which was used to generate a double-frequency output 390 nm (3.2 eV) to pump and a white-light super continuum with a range 450−1000 nm (1.24 to 2.70 eV) to probe. The variable delay between pulses was controlled by means of a motorized stage (Newport IMS600), with the relative transmission change ΔT/T of the probe beam measured using a Si photodetector and lock-in amplifier referenced to the mechanically chopped pump beam. Time-resolved spectra were constructed by analyzing the transmitted probe light of the super continuum by using a monochromator (Bentham). Compensation was made for the chirp of this continuum. Time-Resolved Fluorescence Spectroscopy. The femtosecond time-resolved fluorescence UPC technique can provide time-resolution Δt of up to a few femtoseconds, which is mainly limited by the temporal width Δτp of the laser pulse. The laser source is the same system that was used for the ultrafast TA measurements. (See the previous paragraph.) Our cross-linear fluorescence UPC setup is composed as follows: the pump laser at λex = 390 nm, which is achieved by frequency doubling the 780 nm laser in a BBO crystal, is focused by a lens to a round spot with diameter of 100 μm on the sample at an incident angle of ∼3°. The forward emission is collected by another lens (f = 3) and converged onto another BBO (type I) crystal, in which the emission of spontaneous fluorescence (or SF) is up-converted by the gating beam of 780 nm, through a nonlinear optical interaction, which is called sum-frequency generation (SFG). Then, the UPC beam passes through a double-grating monochromator (JY Gemini) and reaches the detector, a photon counting solar blind photomultiplier tube (PMT). The signal intensity from the PMT is recorded by a gated photon counting technique (Becker & Hickl PMS 400A) with respect to the time delay between pump and gating pulses. The computer-controlled motorized linear translation stage (Newport IMS600) provides the minimum time delay at 8.3 fs/ step. The typical cross-correlation response Δt ≈ 360 fs; this also defines the t = 0 position, which is measured by the SFG of

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MATERIALS AND METHODS The synthesis and characterization data of compound 1 are described in the SI. The syntheses, purification, and 1H NMR data for 2 and 3 are reported elsewhere.3,5 Additional characterization including electrochemical and computational data can be found in the SI. Experimental Conditions. All materials were dissolved in HPLC-grade ethanol (EtOH) for optical spectroscopy studies and performed at ambient temperatures, unless otherwise 6816



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126.8 (C1C(NH2)2), 128.8 (C2,6), 131.7 (C3,5), 138.6 (C4), 166.8 (C(NH2)2).

pump scattering and gating beam; the spectral response bandwidth measured is ∼3 nm by tuning the monochromator only while the UPC crystal angle is being fixed. A fiber-coupled CCD spectrometer (Ocean Optics USB4000), having a spectral resolution of 0.8 nm, is used to simultaneously monitor the backward emission from the sample and record the steady-state spectra. The pump power (P) is varied with a neutral density filter. P = 10 mW means that a single fs pulse has an energy of 100 nJ or 2.0 × 1011 photons; the corresponding pump influence and transient power density are 1.27 mJ/cm2 and 6.37 GW/cm2, respectively. Time-correlated single-photon counting (TCSPC) was also performed as a complement to the UPC. This system is based on a Coherent Mira900-f Ti:sapphire picosecond oscillator producing 1 to 2 ps pulses (fwhm) at 76 MHz. The nearinfrared optical pulses are frequency-doubled in a nonlinear BBO crystal to produce optical pulses in the blue. The TCSPC detection system has been described elsewhere.31 The optical/ electronic instrument response function in the TCSPC experiments (measured form a solution of light scattering Ludox particles) is ∼20 ps fwhm. Data Analysis. The experimental data of the dynamics, both fluorescence and TA, were analyzed using a sum of exponentials I(t) = ∑iAi(λ)·e−kit using a subroutine in Microcal Origin 7.0. In the UPC experiments, data analysis was done using the SFG function by performing the standard method of iterative reconvolution and nonlinear least-squares fitting of the decay with the SFG function in a Microsoft Excel spreadsheet.18 In this procedure, it was necessary to allow a shift of the SFG with respect to the onset of the fluorescence decay profile. This was caused by slight changes in geometry and phase matching in the sum frequency BBO crystal of the gate and fluorescence signals, as compared with the SFG function. The TCSPC data were analyzed using a similar method, as previously described for the UPC data. Computational Details. All calculations were carried out with the Gaussian 09 package32 using the Hartree−Fock (HF) wave functional or the DFT hybrid functionals (B3LYP,33 MPW1K,34,35 or CAM-B3LYP36) and the 6-31G* basis set.37 The optimized S0 geometries of 1, 2, and 3 at B3LYP/6-31G* without symmetry constraints were found to be true minima based on no imaginary frequencies found. A scaling factor of 0.95 was applied to the computed frequencies.38 All rotation barrier energies were estimated using appropriate dihedral angle constraints. Electronic structure and TD-DFT calculations were also carried out on optimized S0 geometries at the B3LYP/631G* level of theory. The MO diagrams and orbital contributions were generated with the aid of Gabedit39 and GaussSum40 packages, respectively. Calculated 13C NMR chemical shifts obtained at the GIAO41,42-B3LYP/6-31G*// B3LYP/6-31G* level on the optimized geometry were referenced to TMS for 13C: δ(13C) = 1.1[185.0 − σ(13C)]. Computed 13C NMR data for 1: δ 38.6 (C8), 109.3 (C1); 119.1 (CN), 119.5 (C3,5), 129.1 (C2,6), 155.9 (C4), 160.9 (C7); 2: δ 15.5 (CH3), 38.1 (C8), 52.9 (CH2), 73.0 (CH), 111.7 (C1), 117.9 (C3,5), 119.5 (CN), 131.7 (C2,6), 154.9 (C4), 167.0 (C7); 3: δ 15.3 (mor-CH3), 18.9 (pip-CH3), 29.2 (pip CH), 38.2 (C8), 33.2 (pip-CH2), 51.7 (mor-CH2), 51.8 (mor-CH2), 54.0 (pip-CH2), 56.3 (pip-CH2), 74.1 (mor-CH), 114.2 (C1), 119.1 (C3,5), 119.6 (CN), 133.7 (C2,6), 154.9 (C4), 173.8 (C7); 4: δ 26.1 (C(CN)2), 116.6 (C4), 118.0 (C2,6), 125.0 (CN), 128.5 (C3,5), 143.2 (C1C(CN)2); 5: δ

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RESULTS The proton NMR spectrum for 1, reported here for the first time, showed two distinct N−H peaks, indicating no free rotations at the C7−N bonds in solution at ambient temperature. This observation suggests a degree of conjugation between the nitrogen lone pairs and the methine C7 atom. Two NH2 peaks were also seen in the 1H NMR spectrum for 2 along with a poorly resolved CH2 multiplet peak (four nonequivalent hydrogens), pointing to restricted rotations at both C7−N bonds. The CH2 peaks in 3 were also complex due to eight nonequivalent hydrogens on the basis of C7−N bond restrictions. No changes in the 1H NMR data for 1−3 were observed in more dilute solutions (∼10−5 M), and thus aggregation effects responsible for the nonequivalent peaks are ruled out. The 13C{1H} NMR spectrum for 1 revealed seven carbon peaks, as expected when C2 symmetry is considered (Figure 1).

Figure 1. 13C{1H} NMR spectra for 1−3.

One carbon peak at 34.7 ppm is assigned to C8 on the basis of the reported value of 27.2 ppm43 for the [PhC(CN)2]− anion 4 (Chart 2). The peak at 165.0 ppm is assigned to C7 as a value of 166.5 ppm is quoted44 for the [PhC(NH2)2]+ cation 5. It may be assumed that compound 1 is a zwitterion with formally [C(NH2)2]+ and [C(CN)2]− groups attached to a benzene ring. The 13C peak difference of between the C7 and C8 peaks is an useful indicator of zwitterionic character present in derivatives of TCNQ with a C7−C8 difference of 130.3 ppm for the zwitterion 1. The 13C peak at 90.8 ppm in the solid-state NMR spectrum for TCNQ is assigned to the equivalent C7 and C8 atoms.45 TCNQ has long been known to be of quinonoid character, where the C1−C7 and C4−C8 bonds are essentially double bonds.46 Figure 1 also shows the 13C NMR spectra for 2 and 3 with C7 peaks at 164.5 and 168.4 ppm and C8 peaks at 32.5 and 34.1 ppm, respectively. The 13C peaks concerning the aromatic unit at 113.2−165.0 ppm are effectively identical for all three compounds 1−3. The peaks at ca. 71 ppm for 2 and 3 correspond to the carbons directly bonded to oxygen. The broad 13C NMR peak at 53.0 ppm for CH2 groups attached to nitrogen in the morpholino group in 2 indicates that the two 6817



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Chart 2. Reported 13C NMR Shifts for the Methine Carbons in TCNQ, [PhC(CN)2]− Anion 4, [PhC(NH2)2]+ Cation 5, and DADQ 6

Table 1. Experimental Steady-State Photophysical Data for 1−3a

1

2

3

CH2 groups are nonequivalent due to restricted rotation at the C7−N(mor) bond in solution. Evidence of restricted rotations at the C7−N bonds in 3 is demonstrated by the two nonequivalent morpholino CH2 peaks and the two nonequivalent piperidino CH2 peaks in the 13C NMR spectrum. Given that many papers6,7,10,11 have reported 13C NMR data for DADQs, there has been only one case where the 13C peaks for the methine carbons have been assigned. In this case, the C7 and C8 peaks were quoted5 at 92.3 and 71.1 ppm for compound 6 (Chart 2), which are considerably different from the peak assignments for 1. This compound 6 is thus presumed to be of quinonoid character based on the C7−C8 chemical shift difference of only 21.2 ppm. Steady-State Optical Spectroscopy. The optical properties of the DADQ 1 are presented here for the first time while some optical and electronic properties of 2 and 3 have been investigated previously.3,5,18 Absorbance (1-T), fluorescence and excitation spectra of the DADQs 1−3 in EtOH solutions are shown in Figure 2, and their data are listed in Table 1. The absorbance spectra for 1 and 2 are similar, whereas the spectrum for 3 has the strong low-energy electronic absorption red-shifted by ∼2100 cm−1. Absorbance and fluorescence spectra for 1−3 were also recorded in acetonitrile and other alcohols (Table 1 and Figure S2 in the Supporting

solvent

absorption (nm)

fluorescence (nm)

Stokes shift (cm−1)

acetonitrile methanol ethanol butanol pentanol octanol decanol acetonitrile methanol ethanol butanol pentanol octanol decanol acetonitrile methanol ethanol butanol pentanol octanol decanol

399 383 388 396 398 402 403 394 377 385 392 396 402 402 436 410 420 429 430 434 436

523 449 450 448 450 452 452 500 485 481 481 483 484 481 540 522 530 525 526 526 522

5943 3838 3550 2931 2904 2752 2690 5381 5906 5184 4720 4549 4214 4085 4417 5233 4941 4262 4244 4030 3779

a Excitation wavelength in the fluorescence was 380 nm for all systems. See SI for spectra.

Information). The lowest electronic absorption maxima for 1−3 in methanol are consistently at ∼500 cm−1 higher in energy compared with ethanol. DADQ 1 shows the smallest Stokes shift also in the extended series of alcohols while 2 has the largest and 3 in intermediate in this respect. Furthermore, it is obvious from the data in Table 1 that for the longer chain alcohols the Stoke shift decreases with a similar trend for all three systems 1-3. Analysis of the photophysical data using the Kamlet−Taft methodology for 1-3 in the same series of alcohols and acetonitrile where performed and plots of absorbance and fluorescence maxima against the α, β and π* parameters were constructed (see SI). Furthermore, plots of the Stoke shift against the polarity parameter π* were also constructed (see SI). It should be noted that a full Kamlet−Taft analysis is not possible to perform as these three DADQs are not soluble in cyclohexane, which is the most commonly used reference solvent in this analysis. It is therefore not possible to calculate absorption maxima and compare these to the experimentally obtained values. However, the limited Kamlet−Taft analysis reveals that the center of weight of the absorption is slightly more sensitive to the dielectric medium as compared to the fluorescence center of weight. Weak fluorescence was observed for all three compounds with single broad spectra for all systems in low viscosity solvents. The weak fluorescence indicates low PLQYs as previously observed for 2 and 3.3,5 The PLQY measurement of 1 in ethanol yields an approximate value in the range of 10−2 − 10−3 due to the very weak fluorescence intensity. Compound 1 display the same temperature dependence as reported for 2 and 3 with the fluorescence intensity increasing 20-fold as the temperature is lowered to 100 K (see SI). In more viscous solvents such as the longer chain alcohols, the intensities of compound 1 also increase. The extent of increase in

Figure 2. Absorbance (black solid line), excitation (green dashed line), and fluorescence (blue solid line) spectra for 1−3 in ethanol. See Table 1 for data and text for details. 6818



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Figure 3. Single wavelength TA dynamics (left) at various probe wavelengths for 1/EtOH (1 in ethanol) with 390 nm excitation. (Dots represent raw data and line is fitted function.) Time-resolved TA spectra for 1/EtOH at various time delays (right) for the same excitation. See Table 2 for data and text for details.

Figure 4. Single wavelength transient absorption dynamics (left) at various probe wavelengths for 2/EtOH with 390 nm excitation. (Dots represent raw data and line is fitted function.) Time-resolved TA spectra for 2/EtOH at various time delays (right). See Table 2 for data and text for details.

fluorescence intensity correlates with viscosity of the solvent as expected; the largest relative increase is observed in the most viscous solvent (decanol). There is a consistent trend for all three DADQ adducts in this respect (see SI for Figures S10 − S12 for the intensity variations). The fluorescence band of 3/EtOH is slightly asymmetric, which indicates more than one emitting species. The fluorescence maximum at 450 nm for 1 has a Stokes shift of 3550 cm−1, which is smaller than the observed Stokes shifts of 5700 for 2 and 4900 cm−1 for 3 in EtOH. For compounds 1−3 in a series of alcohols, the same trend is observed. (See Table 1 and SI for additional data.) The geometry rearrangements involved on going from the ground state to the singlet excited state may be reflected in the observed Stokes shifts, and thus more considerable geometric rearrangements occur in the excited states of 2 and 3 than in 1. Interestingly, this trend is reversed when 1−3 is dissolved in acetonitrile; the Stokes shift for 1 is 6265 cm−1 while 2 and 3 displays Stokes shifts of 5696 and 5289 cm−1. Acetonitrile has a lower viscosity (0.441 cP as compared with 1.074 cP for EtOH) and also different hydrogen bond abilities according to their Kamlet−Taft parameters.

Solvatochromism of these DADQs therefore appears to be complex, and hydrogen bonds may be a contributing factor here. (Additional data on the solvatochromic behavior can be found in the SI). The similarities between the excitation and absorbance spectra found in all three DADQs could imply that the emissive excited states are not affected by hydrogen bonding when monitored through the fluorescence (Figure 2). For the emissive part of the excited-state population, it would be reasonable to expect clear substantial differences between the excitation and absorbance (1-T) spectra if hydrogen bonds between DADQ adduct and solvent existed as electronic excitation would break hydrogen bonds on account of negligible dipole moments in their excited states. However, for the nonemissive part of the excited-state population, the situation can clearly be different and hydrogen bonds between solute and solvent cannot be ruled out. A different scenario to hydrogen bonding is that the zwitterionic character leads to strong solvation in the electronic ground state. This can also (and perhaps better) be rationalized by the results of Figure 2. As the three DADQs, while having similar ground-state electronic properties (large μg), they clearly have different hydrogen-bond abilities, which would be 6819



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Figure 5. Single wavelength transient absorption dynamics (left) at various probe wavelengths for 3/EtOH with 390 nm excitation. (Dots represent raw data and line is fitted function.) Time-resolved TA spectra for 3/EtOH at various time delays (right). See Table 2 for data and text for details.

revealed in the absorbance/excitation spectra. A closer inspection of the excitation and absorbance spectra reveals some differences for 1 and 3 (Figure 2). However, the trend of these differences cannot easily be rationalized in terms of hydrogen bond ability, as 1 and 3 show the same blue shift of the excitation, and these two systems clearly have different hydrogen bond abilities to the solvent (ethanol). It is therefore possible that in parallel to solvation effects inhomogeneous broadening of the absorption caused by a distribution of ground-state conformations prevails. In this situation, different conformers would contribute more to the fluorescence, which would lead to the small differences between absorbance and excitation spectra that is observed for 1 and 3. Photoinduced degradation of 3 has been previously reported47 in studies of the stability of polymer-doped waveguide structures but is not seen on the short time scale of experiments carried out in the course of this work. Indeed, the long-term stability of DADQ doped polymer waveguides in oxygen-free environments suggests that they may have potential for use in OLED applications. Ultrafast Transient Absorption and Fluorescence Dynamics. The TA measurements on the DADQ derivatives 1−3 showed fast dynamics on the picosecond time scale with complicated wavelength dependence, as seen in Figure 3−5. Signals are attributed to stimulated emission (SE) (ΔT/T > 0) and photoinduced absorption (PIA), which may be caused by a photoproduct or excited-state absorption (ESA) to higher energy levels within the DADQ energy level manifold (S1→ Sn), (ΔT/T < 0). For all three DADQs 1−3, the SE is on the blue high-energy side of the TA spectra (ΔT/T > 0) at short delay times. This assignment is also based on the fluorescence of all three systems shown in Figure 2. The SE observed in the TA spectra is well within the steady-state fluorescence for all compounds. Consequently, the TA signals on the red lowenergy side are assigned to ESA or PIA (ΔT/T < 0). A switchover for 1 from the SE to ESA is observed with an estimated recovery time/duration of ∼20 ps (Figure 3 and Table 2) at 500 nm. This SE has similar dynamics as the fluorescence dynamics (Figure 6) and overlaps with the fluorescence (Figure 2). The SE is therefore attributed to the emission from the untwisted system (S1 → S0). However, after ∼5 ps the SE at 500 nm has switched over to PIA, and the recovery phase of this process is considerably slower, as shown

Figure 6. Time-resolved fluorescence of 1−3 in EtOH for 380 nm excitation. The fluorescence is monitored in the respective maxima: 450 nm for 1, 480 nm for 2, and 525 nm for 3. Dots represent raw data and solid lines are fitted functions. See Table 2 for data and text for details.

in Table 2. This phase is likely to be due to the formation of a photo product with distinctly different dynamics and hence the assignment to PIA. The TA dynamics for 1 at 570−650 nm are due to S1 → Sn transitions to higher excited states, and this process is assigned to ESA. The TA dynamics for 2 at 660 nm (Figure 4) is very similar to that of the SE, and this emission can be assigned to ESA with S1 → Sn transitions. Because the SE and ESA maxima are wellseparated in the TA spectrum and the dynamics are similar, no photo products have been formed due to electronic excitation, as evident in the ultrafast TA and fluorescence experiments of 2. In the blue part of the TA spectrum for 3, the SE dynamics are close to the fast phase of the fluorescence dynamics (Table 2). Biexponential decays are observed and the fluorescence spectrum (Figure 2) reveals a more asymmetric spectrum, and this implies that the fast phase originates from a conformational substate. The slower SE dynamics at 550 nm is similar to the fluorescence dynamics at 560 nm, and these processes are therefore attributed to S1 → S0 emission from the untwisted system. The dynamics observed in the red part of the TA spectrum is close to the SE, and fluorescence is detected in the 6820



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Table 2. Fluorescence and Transient Absorption Dataa fluorescence

probe wavelengths λEM (nm)

λTA (nm)

430 480 510

1

τ1(A1) (ps)

transient absorption

τ2(A2) (ps)

3.5 ± 1.0

∼20 ± 10

9.8 8.7 10.2 8.9 ± 1.5

530 660

6.7 ± 1.5

490 525 560

2.6(0.68) 11.5 15.5

12.8(0.32)

4.7 ± 1.0 16.9 ± 1.0

485 550 650 700 a

τPIA (ps)

2.1 ± 1.0 2.4 ± 1.0

465 490 530

3

τESA (ps)

4.0 3.6 3.9 500 570 650

2

τSE (ps)

15 ± 5 3.2 ± 1.0 6.7 ± 1.0

τPIA = photo induced absorption dynamics, τESA = excited state absorption dynamics, and τSE = stimulated emission dynamics.

blue. This process is due to ESA with S1 → Sn transitions. The signal (ΔT/T < 0) appearing after ∼10 ps in the blue part of the TA spectrum is attributed to the PIA of a photo product. For biexponential processes, the total amplitude is normalized to 1.0 Figure 6 shows the results from the UPC fluorescence experiments for 1−3 in EtOH. As a control, the excitation intensities in the fluorescence dynamics experiments were varied by orders of magnitude, but the decay lifetimes were found to be independent of these intensity variations. The fluorescence decays shown in Figure 6 were therefore recorded with high excitation intensities at 1.0 × 1012 photons·cm−2. Additional time-resolved emission experiments were performed on 1−3 on more viscous alcohols using the well-known TCSPC technique. These data clearly show that the fluorescence decays depend strongly on the viscosity of the medium. (See Table 3 and Figures S14−S16 in the SI.) For the case of more viscous solvents (octanol and decanol), it was necessary to fit the data to a biexponential decay. It is likely that the hydrogen-bond network in those media has some influence on the photo physics, leading to a more complex excited-state

decay that subsequently is reflected in the time-resolved fluorescence. Furthermore, in line with the results obtained for 1−3 in ethanol (Figure 6), when comparing DADQ adducts in the same solvent viscosity the data show that the dynamics for 1 is fastest and that for 3 is slowest, with 2 in between. Computations. While the geometry of the DCNQ 1 has not been determined experimentally, the geometries of DCNQs 2 and 3 have been determined by X-ray diffraction.3,16 The optimized geometries in the “gas-phase or vacuum” environment for 2 and 3 at B3LYP/6-31G* were compared here with their experimental geometries. These optimized geometries have considerable quinonoidal character with shorter C1−C7 and C4−C8 bond lengths than the experimental geometries (Table 4), Optimized geometries from alternative methods such as Hartree−Fock (HF), CAMB3LYP, and MPW1K gave even shorter C1−C7 and C4−C8 bonds. The optimized geometry for TCNQ at B3LYP/6-31G* agrees very well with the experimentally determined geometry.46 The “gas-phase” optimized geometries are clearly not appropriate for comparison with experimental data of 1, 2, and 3 due to the zwitterionic nature of these compounds, where these geometries are considerably influenced by external effects.48,49 Optimized geometries of 1 based on different solvents using polarized continuum solvation models50 were carried out to examine the effect of different solvents on the quinonoidal/zwitterionic character of 1. Computed nucleusindependent chemical shift (NICS),51 C4−C8 bond length, 13C GIAO-NMR, IR frequency, and TD-DFT absorption (S0 > S1) data for 1 are listed in Table 5. NICS is a measure of aromaticity, where benzene has an NICS value of −9.7 ppm and TCNQ has an NICS value of −0.3 ppm at B3LYP/6-31G*. An NICS value of −10 ppm would be considered aromatic, whereas 0 ppm is nonaromatic. There are clear trends between bond length, NICS, 13C NMR, and IR values and the polarity of the solvent, indicating that observed 13C NMR and IR data are useful in estimating the degree of aromaticity in DADQs. Very good agreement between the experimental and computed 13C GIAO-NMR

Table 3. Time-Resolved Fluorescence Data for 1-3 in a Series of Alcohols Obtained by TCSPCa DADQ adduct

solvent

1

pentanol octanol decanol pentanol octanol decanol pentanol octanol decanol

2

3

τ1 (A1) ps (a.u.) 16 18 25 26 20 55 38 30 58

(1.0) (0.69) (0.59) (1.0) (0.47) (0.84) (1.0) (0.36) (0.38)

τ2 (A2) ps (a.u.) 46 (0.31) 62 (0.31) 57 (0.53) 507 (0.16) 88 (0.64) 132 (0.62)

a

For bi-exponential decays, the total amplitude was normalized to 1.0. The excitation wavelength in the fluorescence was 380 nm for all systems. See the SI for fluorescence decays and text for details. 6821


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and IR frequency data was found using solvation models with high dielectric constants such as ethanol. The ethanol solvation model was thus applied to the geometry optimizations of 2 and 3. A much better agreement between the experimental3,16 and computed solvation-applied geometries is shown for selected geometric parameters in Table 4. Computed geometries of DADQs using the ethanol solvation models are expected to be in good agreement with other geometries of DADQs determined experimentally.7,8,14 The NICS values for 1, 2, and 3 are −5.8, −6.0, and −5.7 ppm, respectively, using ethanol in the solvation models and confirm the largely aromatic character and thus zwitterionic character of these DADQs. The optimized geometry of 1 applying ethanol as solvent is likely to be determined experimentally. This geometry has a twist angle of 25° between the [C(NH2)2]+ group and the ring plane compared with 2 and 3 at 38° and 43°, respectively. These twist angles in Table 4 increase as the zwitterionic character increases by applying the solvation models. The energy barriers to planarity between the amine groups and the ring are estimated to be 1.3, 6.9, and 6.9 kcal·mol−1 for 1, 2, and 3, respectively, using the solvation model. The rotation energy barriers along the C1−C7 bond, based on a perpendicular [C(NR2)2]+ orientation, are 6.9, 4.2, and 5.4 kcal·mol−1 for 1, 2, and 3, respectively, which show that the energies for planar geometries in 2 and 3 are higher than energies of geometries containing a perpendicular −[C(NR2)2]+ orientation. These values suggest that planar conformers of 1 would be present in solutions at ambient temperatures, while planar conformers of 2 and 3 are less likely. The rotation barriers along the C4−C8 bond, based on a perpendicular [C(CN)2]− orientation, are high at 15.6, 14.7, and 14.7 kcal·mol−1 for 1, 2, and 3, respectively. As two peaks corresponding to nonequivalent protons of the NH2 group in the 1H NMR spectra for 1 and 2 and peaks corresponding to nonequivalent methylene carbons in 3 are observed, rotations about the C7−N bonds are therefore restricted in solutions of 1, 2, and 3. The C7−NH2 rotation energy barriers for 1 and 2 are estimated at 9.6 and 9.9 kcal· mol−1, respectively, on the basis of the perpendicular orientation of the NH2 group using one constrained torsion angle. The observed strong ṽ(CN) bands in the infrared spectra of TCNQ and DADQs have been used to characterize the quinonoid−zwitterionic character of these compounds. TCNQ itself has a ṽ(CN) band of 2220 cm−1, pointing to quinonoid character, whereas compound 1 has two ṽ(CN) bands at 2170 and 2125 cm−1, indicating zwitterionic character.2 The IR spectrum of the [PhC(CN)2]− anion 4 also has two ṽ(CN) bands at 2169 and 2121 cm−1. The IR spectra of the DADQs 2 and 3 also contain similar IR bands to those for 1 and 4. Frequency calculations on 1−4 and TCNQ are in accord with these reported values. (See SI Table S4.) Curiously, compound 6 also has two ṽ(CN) IR bands at 2180 and 2135 cm−1, pointing to zwitterionic character in this case, and this is also in agreement with computed frequency data. Gauge-independent atomic orbital (GIAO) NMR computations were carried out on 1−3, using DMSO as model solvent, as observed NMR data for these DADQs were recorded in d6DMSO. The agreement between observed and computed 13C chemical shifts is excellent, so the peaks shown in Figure 1 are assigned accordingly. Excellent agreement between observed and computed 13C chemical shifts for the methine carbons was also shown in related derivatives, TCNQ, anion 4, and cation 5

25.0 21.0

28.7 29.5

38.7 37.1

45.5(7) 46.2(8)

34.6 35.7

43.5 43.4

45.4(6) 45.4(6)

1.349(5) 1.339(5) 1.355 1.351 1.375 1.367

1.459(6) 1.395(6)/1.401(6) 1.372(6)/1.375(7) 1.397(6)/1.413(6) 1.442(7) 1.456 1.415 1.380 1.423 1.438 1.422 1.431 1.367 1.436 1.410

1.454(8) 1.380(8)/1.399(7) 1.367(8)/1.374(7) 1.400(6)/1.440(6) 1.444(7) 1.319(7) 1.330(8) 1.455 1.415 1.379 1.423 1.438 1.340 1.345 1.448 1.415 1.378 1.423 1.436 1.336 1.405 1.434 1.365 1.438 1.408 1.365

C1−C7 C1−C2/C1−C6 C2−C3/C5−C6 C3−C4/C4−C5 C4−C8 C7−NH2 C7−N(Mor) C7−N(Pip) C2−C1−C7−NH2 C2−C1−C7−N(Mor) C2−C1−C7−N(Pip)

1.415 1.432 1.367 1.436 1.408 1.366

3 (calc, gas) 2 (expt, X-ray)3 2 (calc, EtOH) 2 (calc, gas) 1 (calc, EtOH) 1 (calc, gas) bond or angle

Table 4. Selected Bond Lengths (Å) and Torsion Angles (deg) for Computed and Experimental Geometries of 1, 2, and 3

3 (calc, EtOH)

3 (expt, X-ray)16


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Table 5. Comparison of Selected Data for 1 Applying Gas-Phase and Solvation Models of Different Polarities vacuum argon n-hexane cyclohexane CCl4 CS2 Et2O CHCl3 THF 1-pentanol EtOH MeOH MeCN DMF DMSO water

dielectric constant (ε)

C4−C8 length (Å)

NICS (ppm)

0.00 1.43 1.88 2.01 2.29 2.61 4.24 4.71 7.43 15.13 24.85 32.61 35.69 37.21 46.82 78.36

1.408 1.411 1.416 1.417 1.419 1.421 1.427 1.428 1.431 1.435 1.436 1.437 1.437 1.437 1.437 1.438

−2.7 −3.3 −3.9 −4.0 −4.2 −4.4 −5.0 −5.1 −5.4 −5.7 −5.8 −5.9 −5.9 −5.9 −5.9 −6.0

13

C (C8) (ppm) 61.6 56.5 52.4 51.4 50.3 48.6 44.8 44.1 41.9 39.9 39.2 38.8 38.8 38.7 38.6 38.3

sym ṽ(CN) (cm−1)

asym ṽ(CN) (cm−1)

TDDFT S0 > S1 (nm)

2205 2199 2195 2193 2192 2189 2184 2182 2179 2176 2174 2174 2174 2173 2173 2174

2185 2176 2170 2168 2166 2163 2155 2153 2147 2140 2140 2139 2138 2138 2138 2137

387 401 408 410 410 414 401 404 398 392 389 387 387 389 388 385

with computed values of 88.4, 26.1, and 166.8 ppm, respectively43−45 (Chart 2). Although there are reported 13C NMR data for several DADQs,7,8,11 their peak assignments have not been carried out. Only one DADQ, 4-{[CH3N(CH2CH2)2N]2C}-1-[(NC)2C]-C6H4 7, in the literature so far has the expected number of 13C NMR peaks, and these are assigned here. Calculated NMR data on 7 showed very good agreement with observed data and are thus assigned. (Chart 3) Chart 3. Calculated 13C NMR Chemical Shifts and Peak Assignments Carried out on Observed 13C NMR Chemical Shifts for DADQ 7

Figure 7. Frontier orbitals for 1 with MO contributions (%) from the three groups: −C(NH2)2, −C6H4−, and −C(CN)2.

involving long-range distances correctly, but TD-DFT data from this CAM-B3LYP functional gave similar results to those from the B3LYP functional. Nevertheless, the agreement between observed and S0 > S1 computed maxima of 388 and 389 nm for 1 is very good, and this absorption arises from a HOMO > LUMO transition. This transition is not entirely charge transfer in nature due to considerable MO overlap between the HOMO and LUMO at the aromatic ring, where the ring contributes to 30% of the HOMO and 41% of the LUMO. This π > π* ring transition is responsible for the strong absorption observed experimentally and for the calculated high oscillator strength of f = 0.76 at B3LYP/6-31G* (352 nm, f = 0.88 with CAM-B3LYP/6-31G*). Similar TD-DFT results for 2 and 3 are as expected based on the computed HLG energies with calculated absorption wavelengths of 403 and 420 nm for 2 and 3, respectively. Simulated absorption spectra from TD-DFT data of 1−3 are shown in Figure 8. The higher energy electronic absorption at 250 nm are assigned HOMO > π*(ring) and π(ring) > LUMO transitions in 1. The occupied molecular orbital (MO) containing the “lone-pair” nitrogens is at −8.35 eV (HOMO−3) in 1, at −7.09 eV (HOMO−1) in 2, and at −6.81 eV (HOMO−1) in 3. The local transition from the lonepair nitrogens to the cationic carbon LUMO thus became lower in energy on going from 1 then 2 to 3. The computed absorptions at 250 nm for 2 and 3 arise from HOMO > π*(ring) and N(lone pair) > LUMO transitions based on the TD-DFT data. The absorptions at 250 (shoulder) and 272 nm in the observed data for 2 and 3 (Figure 2) are assigned the

Because GIAO-NMR computations are demonstrated here to be accurate, compound 6 was looked at due to the unusual 13 C chemical shifts of 92.3 and 71.1 ppm reported in d6-DMSO for C7 and C8, respectively.6 The calculated chemical shifts for C7 and C8 of 6 are 150.7 and 44.7 ppm, respectively. Given the excellent agreement between the observed and computed frequency data for 6 (see Table S4 in the SI), it is concluded here that compound 6 is zwitterionic, like all other DADQs discussed here. The reported 13C peak assignments for 6 need revision, as the calculated C8 peak shift is close to the 13C solvent peak of d6-DMSO at 40.2 ppm, and the observed C8 peak is probably obscured by this solvent peak. Electronic structure calculations on 1 reveal that the HOMO is largely localized on the C(CN)2 group and the LUMO is somewhat delocalized on the C(NH2)2 group and the ring, as shown in Figure 7. TD-DFT data at B3LYP/6-31G* on 1 do not appear to follow the trends found for IR and NMR data when different solvent models are applied, as shown in Table 5. Charge transfer is not generally modeled correctly with B3LYP.52 CAM-B3LYP is known to model charge transfer 6823



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perpendicular geometries are probably present in solution at the ground state. It was necessary to apply geometry constraints for the planar geometries in the excited state for 1−3, as such geometries could not be located from planar starting geometries on optimization without such constraints. The planar excited-state geometries were obtained by using symmetry constraints C2v (for 1) and Cs (for 2 and 3). The predicted S0 < S1 emission maxima values for the twisted forms are between 1214 and 1269 nm for 1−3 with perpendicular C(CN)2 groups and between 1567 and 3294 nm for 1−3 with perpendicular cationic amine groups. The oscillator strengths (f) in these emissions are computed to be negligible. Such data suggest that the dominant nonradiative pathways in these DADQs 1−3 arise from these twisted systems. For the planar geometries, the predicted emission maxima are 442 nm for 1, 502 nm for 2, and 535 nm for 3 with high oscillator strengths of 0.8886 (1), 0.9777 (2), and 1.0425 (3). These planar excited-state geometries are probably responsible for the observed emissions in these DADQs (Figure 2), and simulated emission spectra of 1−3 are shown in Figure 8. It has been shown from the steady-state and ultrafast fluorescence measurements of 3 here that two excited-state geometries may be present. One may be the planar excited-state geometry, and the other could be the fully optimized excited state geometry with twist angles of ca. 34°, which has a predicted S0 < S1 emission maximum of 505 nm with f = 0.8915.

Figure 8. Simulated absorption and emission spectra for 1−3 in ethanol. From top to bottom: 3, 2, and 1. The emission spectra are generated from constrained planar excited-state geometries.

HOMO > π*(ring) and N(lone pair) > LUMO mixed transitions here. Excited-state geometries were looked at computationally here to aid interpretations of the observed fluorescence data. Using the optimized ground-state geometries of 1−3 as starting geometries, the excited-state geometry optimizations were carried out at TD-DFT B3LYP/6-31G* without any geometry constraints and with ethanol as solvent in the applied solvation model. Both optimized excited-state geometries for 1 and 2 had the plane of the C(CN)2 groups perpendicular to the aromatic ring plane, while the cationic groups remained largely unchanged (Figure 9). The optimized excited-state geometry

■

DISCUSSION For 2 and 3, the ground-state geometries are not coplanar with respect to the dihedral angle between the electron-donating groups and benzene ring and are zwitterionic in nature, as deduced from the crystal structures.3,16 These geometries are in accord with optimized geometries provided that appropriate solvation models are applied. These models can be based on the computed evolution of the dipole moment and polarizabilities in applied fields.5 This type of analysis has not been performed for 1−3 as it is slightly beyond the scope of this work; however, we expect a similar trend to be followed. Calculations imply that the geometry of 1 will have a similar twisted ground state form as 2 and 3, but the dihedral angle of 1 in the solid or liquid phase is expected to be slightly smaller for an averaged conformer compared with 2 and 3. The calculated rotational energy barriers are small for planarization and could explain why the absorption spectra are broad and devoid of any fine structure. Optical (and electronic) excitations on the ground-state geometries for 2 and 3 result in alterations of the dihedral angle toward a more planar configuration of the molecular moieties. The same process is assumed to occur in 1. The change in conformation due to optical excitation is, in principle, a friction-dependent process, where the medium, that is, the solvent, and the size and extent of functional side groups of the solute will influence the process.3,5,17 The viscosity of the medium affects the planarization of the DADQ adduct structure. As a consequence, the radiative decay will be in competition with the nonradiative decay of the excited state through conformational change. The conformational change and the subsequent relaxation process are therefore reflected in the fluorescence dynamics of the individual system. Indeed, the obtained fluorescence dynamics for the different DADQ

Figure 9. Excited-state geometries for 1 obtained from different starting geometries of 1 without geometric constraints.

for 3 was similar to the initial starting geometry with the C(CN)2 group planar with the aromatic ring and smaller twist angles of 34.1 and 34.5° for C2−C1−C7−N(Mor) and C2− C1−C7−N(Pip) respectively. Starting from a geometry of 3 with the −C(CN)2 group perpendicular to the aromatic ring, an excited-state geometry optimization without geometry constraints gave a geometry with the −C(CN)2 group still perpendicular to the aromatic ring. From the starting geometries where the amine groups are perpendicular to the aromatic ring in 1−3, excited-state geometries of the same conformations were also located without any geometric constraints imposed, and the solvation model was applied (Figure 9). Such excited-state geometries would be likely to be formed experimentally, as these 6824



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3, which, in turn, is due to the radius of the functional groups of 1−3, with the added complication of possible hydrogen bond interactions for 1, as previously discussed. For larger functional group size the internal steric hindrance will influence the dynamics of conformational change more than the medium viscosity. One aspect of this work is the implication of H-bond interaction between solvent and solute. H-bond formation has a considerable impact on the optoelectronic properties of the hydrogen donating/accepting chromophore due to its electrostatic nature.21,22 One important point is the development of the dipole moment of H-bonded chromophores during electronic excitation. Many cases have been examined where the chromophore undergoes an increase in the dipole moment upon excitation, that is, μg < μe; however, the fate of H bonds during electronic excitation can be very different25−27 and can subsequently be reformed again in the excited state. A further complication in this context is that the electronic excitation where μg < μe will lead to different degrees of classical solvation. All of these processes will have a unique impact on the electronic structure of the chromophore and thereby on the optical signatures. In the DADQ excited state, hydrogen bond interactions between solvent and solute are unlikely because the small μe would not facilitate this. The linear dependence of fluorescence rates against the inverse viscosity clearly seen for 2 and 3 (Figure 10) confirms that must be the case; for 1, there are some deviations as previously discussed. Because those alcohols have different hydrogen bond abilities, it would be reasonable to expect significant deviations from the linear behavior if excited-state hydrogen bonding between solvent and DADQ adduct prevailed. This is clearly not observed in Figure 10. This would therefore suggest that the conformational change in the excited state is not hindered or affected by intermolecular interactions; instead, it must be limited by the friction of the solvent exclusively. The DADQ derivative is therefore chemically inert in the excited state, and, to our knowledge, there are few classes of materials with such properties. While the computed data using solvation models are in excellent agreement with observed data in this study, it should be pointed out that H bonds are not modeled here. It remains to be seen how hydrogen bonds are involved in these DADQs. As far as we are aware, there have not been any computational hydrogen-bond, gas-phase, and solvation data to compare with experimental data and determine whether hydrogen bonds play an important role. Calculated 13C NMR and IR spectroscopic data using appropriate solvation models are in excellent agreement with observed 13C NMR and IR spectroscopic data for DADQs 1−3, and thus calculations can be used to accurately determine the quinonoidal/zwitterionic character of each DADQ using the NCIS value and indeed the DADQ geometry likely to be found experimentally. The computed 13C GIAO-NMR data, in particular, are very helpful in assigning observed 13C NMR peaks of DADQs such as 7, the related anion 4, and cation 5. It is shown here for the first time that the 13 C NMR peak of C8, that is, the methine carbon in C(CN)2 in a typical zwitterionic DADQ derivative, would be expected at a high field in the region of 30−50 ppm, where aliphatic carbon peaks are typically found, as demonstrated in Figure 1. The rich possibilities available to tailor the functional groups of the DADQ derivative mean that we can control ground-state intermolecular interactions with almost any material. However, in the excited state, the intermolecular interactions of DADQ

adducts in media with different viscosity confirms this hypothesis. The previously not studied 1 shows the same response to low temperatures and viscous media as 2 and 3:3,5 an increase in fluorescence intensity (by consequence also PLQY) and slower fluorescence decay. (See the SI.) A closely related process in this context to the nonradiative excited state relaxation via conformational change is cis−trans isomerization and planarization processes in charge-transfer systems, which has been shown to be a viscosity-dependent process.53−62 In the same context, planarization of the DADQ derivative structure costs energy, and the height of the barriers is of a similar scale as that found in many of the classical cis− trans isomer systems.54,55 There is a correlation between the estimated energy barriers and the functional group diameter of each DADQ derivative, with increasing barrier height for larger functional group size. The diameter across the donor group, assuming a flat structure, is 5.7 ± 0.2 Å for 1, 7.6 ± 0.2 Å for 2, and 9.4 ± 0.4 Å for 3. As can be seen from the data in Tables 2 and 3, the observed fluorescence lifetimes for the different DADQ derivatives show a correlation with functional group size. Furthermore, Figure 10

Figure 10. Rate constants of the fluorescence plotted against the inverse of the viscosity for 1−3 in ethanol, pentanol, octanol, and decanol. For biexponential decays, the dominant phase of the decay is used here. Symbols represent experimental data. The dashed lines represent guides for the eye. See Table 3 for data and SI for decays.

shows the relationship between the fluorescence rate constants and the inverse of the viscosity (kf ≈ η−1) for 1−3 in alcohols. DADQ adducts 2 and 3 display a clear linear dependence, which is expected for a system where conformational change, limited by the medium viscosity, competes with the fluorescence dynamics.55 For 1, there are clearly deviations from the linearity, as indicated by the trend line (Figure 10). With the two amine groups, the hydrogen bonding ability of 1 is the largest of the three DADQ adducts considered here, and it is therefore very likely that different degree hydrogen-bond interactions across the series of 1/alcohols influence the fluorescence dynamics here. Figure 10 also shows the contrasting behavior of the three DADQ adducts with respect to viscosity dependence. The largest dependence on the viscosity was shown by 1, while 3 showed the least dependence and 2 was intermediate, as seen by the slope of the different trend lines in Figure 10. This is a reflection of the different degrees of steric hindrance within 1− 6825



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derivatives are essentially nonexistent and the fluorescence is therefore modulated by the extent of free volume in the solvent medium; in this context, it becomes an inert optical probe. The fluorescence of functionalized DADQ derivatives therefore has the potential to be utilized as a sensing tool in biological spectroscopy or surface characterization. One application where functionalized DADQ derivatives could be used is, for instance, in viscosity measurements.

the Micro- and Macroscopic Optical Non-Linearity of Zwitterionic Molecules. Opt. Mater. 2003, 21, 29−37. (5) Bloor, D.; Kagawa, Y.; Szablewski, M.; Ravi, M.; Clark, S. J.; Cross, G. H.; Pålsson, L.-O.; Beeby, A.; Parmer, C. and Rumbles, G. Matrix Dependence of Light Emission from TCNQ Adducts. J. Mater. Chem. 2001, 11, 3053−3062. (6) Abd-Elatif, F. F.; El-Shaieb, K. M.; El-Deen, A. G. A General and a Simple Synthesis of 4-[(Z)-4-(Arylimino)-3,4-dihydroquinazolin2(1H)-ylidene]cyclohexa-2,5-dien-1-ylidene Malononitrile from the Reaction of 2-Amino-N′-arylbenzimidamides with 7,7,8,8-Tetracyanoquinodimethane. J. Chem. Res. 2010, 449−451. (7) Jayanty, S.; Radhakrishnan, T. P. Enhanced Fluorescence of Remote Functionalized Diaminodicyanoquinodimethanes in the Solid State and Fluorescence Switching in a Doped Polymer by Solvent Vapors. Chem.Eur. J. 2004, 10, 791−797. (8) Sharma, S.; Radhakrishnan, T. P. Control of Crystal Packing in SHG Active 7,7-Bis(n-pentylamino)-8,8-dicyanoquinodimethane by Intermolecular H-Bonds and Alkyl Chain Interactions. Mol. Cryst. Liq. Cryst. 2000, 338, 257−269. (9) Ravi, M.; Cohen, S.; Agranay, I.; Radhakrishnan, T. P. Molecular and Crystal Structures of a Class of Push-Pull Quinonoid Compounds with Potential Nonlinear Optical Applications. Struct. Chem. 1996, 7, 225−232. (10) Jayanty, S.; Gangopadhyay, P.; Radhakrishnan, T. P. Steering Molecular Dipoles from Centrosymmetric to a Noncentrosymmetric and SHG Active Assembly using Remote Functionality and Complexation. J. Mater. Chem. 2002, 12, 2792−2797. (11) Patra, A.; Hebalkar, N.; Sreedhar, B.; Radhakrishnan, T. P. Formation and Growth of Molecular Nanocrystals Probed by their Optical Properties. J. Phys. Chem. C 2007, 111, 16184−126191. (12) Chandaluri, C. G.; Radhakrishnan, T. P. Amorphous-toCrystalline Transformation with Fluorescence Enhancement and Switching of Molecular Nanoparticles Fixed in a Polymer Thin Film. Angew. Chem., Int. Ed. 2012, 51, 11849−11852. (13) Chandaluri, C. G.; Radhakrishnan, T. P. Hierarchical Assembly of a Molecular Material through the Amorphous Phase and the Evolution of its Fluorescence Emission. J. Mater. Chem. C 2013, 1, 4464−4471. (14) Ravi, M.; Rao, D. N.; Cohen, S.; Agranat, I.; Radhakrishnan, T. P. Solvate-Switchable Powder Second Harmonic Generation in a PushPull Quinonoid System. J. Mater. Chem. 1996, 6, 1853−1855. (15) Cole, J. M.; Copley, R. C. B.; McIntyre, G. J.; Howard, J. A. K.; Szablewski, M.; Cross, G. H. Charge-Density Study of the Nonlinear Optical Precursor DED-TCNQ at 20 K. Phys. Rev. B 2002, 65, 125107−1−125107−11. (16) Kagawa, Y.; Szablewski, M.; Ravi, M.; Hackman, N. A.; Cross, G. H.; Bloor, D.; Batsanov, A. S.; Howard, J. A. K. Polar TCNQ Adducts for Nonlinear Optics. Nonlinear Opt. 1999, 22, 235−240. (17) Kagawa, Y.; Takada, N.; Matsuda, H.; Yase, K.; Halim, M.; Samuel, I. D. W.; Cross, G. H.; Bloor, D. Photo- and Electroluminescence for TCNQ-Amino Adducts. Mol. Cryst. Liq. Cryst. 2000, 349, 499−502. (18) Pålsson, L.-O.; Vaughan, H. L.; Smith, A.; Szablewski, M.; Cross, G. H.; Roberts, A.; Masutani, A.; Yasuda, A.; Beeby, A.; Bloor, D. Guest-Host Interactions between Dichroic Dyes and Anisotropic Hosts. J. Luminesc. 2006, 117, 113−122. (19) Chandaluri, C. G.; Patra, A.; Radhakrishnan, T. P. Polyelectrolyte-Assisted Formation of Molecular Nanoparticles Exhibiting Strongly Enhanced Fluorescence. Chem.Eur. J. 2010, 16, 8699−8706. (20) Pålsson, L.-O.; Nehls, B. S.; Galbrecht, F.; Coombs, B. A.; Dias, F. B.; Farrell, T.; Scherf, U.; Monkman, A. P. Synthesis, Excited State Dynamics, and Optical Characteristics of Oligophenyl-Based Swivel Cruciforms in Solution and Solid State. J. Phys. Chem. B 2010, 114, 12765−12776. (21) Aloise, S.; Pawlowska, Z.; Ruckebusch, C.; Sliwa, M.; Dubois, J.; Poizat, O.; Buntinx, G.; Perrier, A.; Maurel, F.; Jacques, P.; Malval, J. P.; Poisson, L.; Piani, G.; Abe, J. A Two-Step ICT Process for Solvatochromic Betaine Pyridinium Revealed by Ultrafast Spectrosco-

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CONCLUSIONS The excited-state dynamics of the DADQ derivatives described in this work in low viscosity media are limited by the twisting dynamics of the molecular moieties that coordinates the functional group. By introducing limitations on the twisting motions, the radiative decay of the excited state can be enhanced. Because the excited-state dipole moment of DADQ is close to zero, intermolecular interactions in the excited state cannot be facilitated and thus render DADQs to be chemically inert systems in the excited state. Computational data on DADQs are in very good agreement with available geometric and spectroscopic data provided suitable solvation models are applied to these zwitterionic derivatives. In particular, calculated 13C NMR data can be used to aid peak assignments of observed 13C NMR spectra of DADQs, as demonstrated here for the first time.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedure for compound 1 as well as additional spectroscopic and structural data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*M.S.: Tel: +44-(0)191-3343686. Fax: +44-(0)191-3343585. Email: [email protected]. *L.-O.P.: Tel: +44-(0)191-3342135. Fax: +44-(0)191-3342051. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Durham University and EPSRC is gratefully acknowledged. We thank Professor David Bloor for many useful comments on the work in this and related projects. We also thank Durham University for access to its High Performance Computing facility.

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REFERENCES

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